A light emitting diode (briefly called LED) is extensively applied to many environments such as building lighting, vehicle head lamps and tail lamps, backlights and flash lights of liquid crystal display devices including personal computers and high-definition television sets. As compared with conventional light sources such as incandescent lamps and fluorescent lamps, LEDs have distinctive advantages such high efficiency, good directivity, excellent color stability, high reliability, long service life, small size and safe environment.
Generally, a plurality of LEDs are arranged into an LED string, and in many environments, for example, a plurality of LED strings are used in parallel in backlights and flash lights of liquid crystal display devices including personal computers and high-definition television sets.
In use of the LEDs, the corresponding LEDs in each LED string might be short circuited.
Hence, it is desirable to timely, correctly and conveniently detect the short circuit of LEDs in the LED string.
FIG. 1 is a schematic view illustrating detection of LED short circuit in the prior art. For the purpose of brevity, only one LED string 101 is shown. In this prior art, a voltage at a drain of an MOSFET of a switch 102 controlling turn-on or turn-off of the LED string is directly detected and compared with a comparison threshold. If the voltage on the drain is higher than the comparison threshold, it indicates that there are short circuited LEDs in the LED string; if the voltage on the drain is lower than the comparison threshold, it indicates that there are no short circuited LEDs in the LED string.
However, the MOSFET drain is a high-voltage pin, and sometimes its voltage will reach 100 V. Hence, an area of a chip is substantially expanded if units for detecting short circuit of LEDs in the LED string are integrated in a control chip 103. Besides, since there are a plurality of LED strings and each LED string needs to be detected to see whether the LEDs therein are short circuited, the number of pins of the control chip 103 will be increased by detecting the voltage on the MOSFET drain to detect whether there are short circuited LEDs in the corresponding LED strings.
Additionally, due to manufacture differences among LEDs, for the same current, the voltage drops are rather different among the LED strings. In other words, for the same current output, currents might be rather different among the LED strings. This might cause a wrong result in detecting short circuit of LEDs in the LED string. Therefore, before the LED strings are applied to specific environments, it is optimal to perform match detection of the LED strings and to apply matching LED strings, namely, LED strings whose voltage drop difference is less than a comparison threshold, to the same specific environment.